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Halton Arp's Discordant Redshifts

Disclaimer: The presence of this analysis of Arp's work on this site does not mean that I regard Arp as a creationist. Rather, Arp's work has been cited numerous times by Creationists as evidence of everything from controversy and cover-up in mainstream cosmology to support for particular Creationist cosmology claims.

Surprisingly large number of galaxies show evidence some kind of disruption. The question is how many of these disruptions create an apparent alignment with a background object.

Compare the number of objects actually found to the sizes of galaxy catalogs. At present, Arp's counts are almost certainly far lower than what you expect from chance alignments between foreground galaxies and background quasars.

Some questions to think about when examining Arp's claims:
  • How many times are there aligned quasars that don't include a bridge?
  • How many times are there bridges that don't include a quasar?

As a visual example of how perceptions can distort one's sense of statistics, consider the case of the galaxy UGC 10214, aka the Tadpole Galaxy (UGC 10214 data at Simbad).
Examine this galaxy from a ground-based image.
What would you say is the probability of a quasar being aligned along the protrusion?
Image via Simbad POSSII/F/DSS2. This link will launch a Java Viewer application on your local machine.
POSSII/F/DSS2 image of UGC 10214
Now examine a subset of this view in this Hubble image taken with the HST/ACS. Notice all the background galaxies visible in this image which are almost unnoticeable in the image above.
Now what would you say is the probability of a quasar being aligned along the protrusion?
Higher resolution version of this image at STSci.
HST ACS image of UGC 10214

All the galaxies now visible in the background, many of which fall along the protrusion, significantly alters the view of just how many quasars may also fall along this track. Because any background quasars in any image are significantly brighter than the galaxies at the same distance, they are more likely to be visible in the more magnitude-limited image.

I've not found a paper that indicates the presence or lack of quasars along this stream. If anyone knows of such a paper, please .

So here is my current list of questions for Dr. Arp:

  1. Would Arp claim that all these background galaxies are actually nearby ejecta from UGC 10214? What about the background galaxies in these other deep Hubble images: M 101, NGC 1309, NGC 3370?
  2. If all of these other galaxies are not ejecta from the foreground galaxy, how do we distinguish the foreground objects from the background objects?
  3. If Arp wishes to claim these background galaxies really are background galaxies, how does he explain his claims that these types of alignments are low-probability events (less than one in a million)?

Here is another example. Consider the FULL Hubble field of view for the NGC 4319 and Mrk 205 pair and compare it on the same scale as the Hubble Deep Field image (Images from STSci).
Hubble Deep FieldNGC 4319 and Mrk 205
Hubble Deep Field image NGC 4319 and Mrk 205
What these two images tell us is that if NGC 4319 were given an exposure equivalent to the Hubble Deep Field, then odds are good there would be many distant galaxies in the field of view. If any one of those galaxies were an AGN with the jet pointed in our direction, it would be far brighter than the other galaxies at the same distance, and would be visible in a shorter exposure.

Arp's discordant redshifts are another product of the Malmquist bias. When we look at the night sky on Earth, the stars we see are not average stars, but the really bright ones very far away which can outshine the nearby fainter stars. While the bright stars are fewer in number relative to the average stars, the fact that they can be seen from a greater distance means we sample a larger volume of the sky, which more than makes up the difference.